Is it safe to share un-drm'd ebooks?

[any ANONYMOUS forum user]

I have purchased some Kindle ebooks from Amazon, and removed the DRM so that I can read them on my Sony ebook reader. I used Calibre to do the conversion to Epub. I would like to share them with some of my friends, but I am well aware that as soon as I give them to anyone else, what happens to them after that is out of my control. If someone down the line posts them to a newsgroup or a torrent site, could there be any hidden metainfo within the file that would allow Amazon to identify the original purchaser of the ebook (me)? I have heard that Calibre strips all metadata when it converts books, but I would like to be sure.

It is safe NOT to sure undrm'ed ebooks

EPUB internally uses XHTML or XML to represent the text and structure of the content document, and a subset of CSS to provide layout and formatting. XML is used to create the document manifest, table of contents, and EPUB metadata. Finally, the files are bundled in a zip file as a packaging format.

So you can rename an .EPUB file into .ZIP file, extract its content and see for yourself using any text editor.